Refactor function name (#11210)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

rag/utils/minio_conn.py

@@ -92,7 +92,7 @@ class RAGFlowMinio:
                 logging.exception(f"Fail to get {bucket}/{filename}")
                 self.__open__()
                 time.sleep(1)
-        return
+        return None
 
     def obj_exist(self, bucket, filename, tenant_id=None):
         try:
@@ -130,7 +130,7 @@ class RAGFlowMinio:
                 logging.exception(f"Fail to get_presigned {bucket}/{fnm}:")
                 self.__open__()
                 time.sleep(1)
-        return
+        return None
 
     def remove_bucket(self, bucket):
         try: